1. Field of the Invention
This invention relates to a process for producing a fluorine-containing block copolymer, and more particularly to a process for producing a fluorine-containing block copolymer having a good solvent resistance.
2. Description of the Prior Art
It is easy to obtain a copolymer of tetrafluoroethylene, vinylidene fluoride and perfluoro(methyl vinyl ether) of a high vinylidene fluoride content type, for example, in a molar ratio of 10/70/20 in a high yield by polymerization based on one charging of all the amounts of the individual monomers (see COMPARATIVE EXAMPLE 2, which follows). However, copolymers of low vinylidene fluoride content type having a good solvent resistance such as a high resistance to methanol, etc. have been in an increasing demand in the present market.
Even if a copolymer of low vinylidene fluoride content type, for example, in the above-mentioned molar ratio of 45/25/30, is tried to obtain by polymerization based on one charging of all the amounts of the individual monomers, there has been such a problem that vinylidene fluoride having a high polymerization reactivity preferentially undergoes the polymerization, while leaving the tetrafluoroethylene and the vinyl ether, which are hard to copolymerize, as residual gases, and the polymerization reaction is finally discontinued in yield of about 60%, resulting in a poor productivity (see COMPARATIVE EXAMPLE 1, which follows).
To overcome the problem, it is possible to increase the amount of a polymerization initiator or add the polymerization initiator in a divided manner from time to time during the polymerization, thereby increasing the yield, but when the polymerization initiator is used too much, the physical properties of vulcanized moldings, particularly compression set as a criterion for the sealing characteristics, will be deteriorated.
It is also possible to increase the yield by polymerization based on divided addition of the individual monomers, but in that case the polymerization initiator is used in an amount as small as that for the polymerization based on one charging of all the amounts of the individual monomers and an amount of the vinyl ether having a somewhat low reactivity is increased to decrease the vinylidene fluoride content, whereby the time for the divided addition will be prolonged and consequently the working time and polymerization time will be prolonged, resulting in an impractical productivity.